Cob: Oklahoma thoughts and concerns

[Shannon C. Dealy]

On Wed, 7 Aug 2002 swboltin at excite.com wrote:

[snip]
> I was curious, I live in Houston and 80+ is common at night.  Accompanied
> of course with our high humidity.  Is this an issue with the stability of
> cob or just factor in with the comfort level?  This would be something
> that I would definitely like to know prior to jumping in.   Also, the

Humidity should not be a problem, it takes alot of water to significantly
soften cob, and that would require that alot of very humid air pass
through the cob and condense out it's moisture in order to cause any
problems.  I suspect (but have no proof) that if the humidity is not
sufficient throughout a given area to cause the earth below your feet to
remain soft (and it's not due to a high water table), then it shouldn't be
a problem for cob.

> developement I am in, my house included, is developing problems in
> relation to driveways and in some instances foundations, all of the slab.
> It seems that the soil wasn't compacted enough and cracks are developing.
> There are a couple of driveways that have a large portion of the soil
> underneath has washed out and are being supported by re-bar alone.   I
> have read some of the other emails stating that with cob, soil compaction
> is not an issue, and in fact is often avoided.  How does a wall that is
> all one piece get around this?   Any help any of ya'll can shed on either
[snip]

I suspect that the problems you are seeing are not really due to lack of
soil compaction.  I used to live in the Dallas area, and there were alot
of similar problems, but they were caused by things like frost heave and
expansion/contraction due to the moisture content of the soil.  Both of
these are essentially water problems, frost heave occurs when water in the
earth freezes which causes it to expand with tremendous force (this can
move buildings and crack reinforced concrete walls), the other problem is
usually a case of the ground absorbing water during the rainy season which
causes it to expand like a sponge, and then drying out during the hot
summer which causes it to shrink.  Soils with a high clay content are
particularly susceptible to this as some types of clay can absorb
tremendous amounts of water, but pretty much all soils will undergo some
expansion and contraction along these lines.  These are at least part of
the reason that slab foundations are so common in the Dallas (and I assume
Houston) area, by putting the house on a slab, you have essentially built
it on a raft that floats of top of the constantly shifting soil,

How do you prevent/deal with this type of problem?  Basically, you want to
get the water away from the house, if you put in drainage around the
house in the form of drain pipe under a band of gravel surrounding the
house (which extends down below the frost line), it will help to dry the
soil out so that it doesn't expand when it freezes, and even if it does,
since your house is surrounded by gravel which won't hold the water, the
expanding earth will simply push against your gravel which will absorb the
pressure without damage to your building.  As far as expansion and
contraction of the soil due to water content, in many areas this is not
much of a problem, or the expansion/contraction is uniform (it acts slowly
and on the whole building uniformly), so just dealing with the frost heave
issue may be enough.  If water expansion is an extreme problem, you
might possibly be able to solve it by simply going down deeper with your
gravel trenches in order to keep the soil under your house drained so it
can't expand, but there may be issues of seasonal underground streams
which could expand the soil despite your best attempts at drainage.  In
these cases, I think you you have two options, go back to the slab
foundation (engineered to support the weight of a cob building), or
consult an expert who can evaluate your site.

Shannon C. Dealy      |               DeaTech Research Inc.
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